1. Technical Field
This invention generally relates to methods and devices for applying solder to workpieces and more particularly to a method for utilizing a plasma gas mixture containing Argon and CF.sub.4 in a vacuum to clean and coat a conductor.
2. Background
Electronic fabrication processes include the attachment of various electrical elements, including discrete components, resistors, transistors, diodes, switching arrays and integrated circuits such as memory devices, microprocessors, and the like, together with a variety of other circuit elements such as transformers, connectors and heat sinks to a substrate such as printed circuit board which electrically connects and mechanically supports the variety of circuit elements.
Soldering is the most common process used for connecting the variety of circuit elements to the printed circuit board both physically and electrically, fixing them in position. Soldering involves the use of a low melting point metal alloy, usually of a lead-tin type, that when exposed to heat, typically at temperatures around 450.degree. F., melts and flows between adjoining metal surfaces or contacts, joining the adjacent surfaces together.
Most soldering processes include three basic steps: pre-cleaning and deoxidizing, solder reflow or reflow joining, and post-cleaning of residue. Pre-cleaning and deoxidizing are usually accomplished by applying a flux material formulated to remove contaminants and oxides from the surfaces to be soldered. Soldering joins the surfaces to be soldered when the solder is reheated beyond its melting point. Oxides, typically with a higher melting point than solder, can form a barrier and prevent wetting of the surfaces to be soldered if they are not removed prior to the solder reflowing. Residue cleaning involves the removal of flux residue from the cleaning and deoxidizing step.
As the processing speeds of electronic devices increase it will be increasingly important to provide fabrication processes that maximize electron exchange properties in circuits. A flux residue with even moderate electromigration properties can cause extreme difficulties in high speed devices.
In addition, it has been observed that handling of components and assemblies, during fabrication and rework of electronic components, including BGA packages and chip scale packages (CSP) which have re-workable defects such as solder shorts or low volume solder, generally increases the organic contamination on component parts and assemblies. Therefore, a need exists not only to de-oxidize package conductors, but to clean such organic contamination from the electronic components prior to reuse.
Dry or fluxless soldering methods have eliminated the need for the use of a flux in the soldering process and therefore, to a great extent have eliminated the post-cleaning or residue cleaning step. Dry or fluxless soldering methods may theoretically replace the pre-cleaning and de-oxidizing steps in the soldering process. It is still necessary however, to deoxidize conductive surface to be soldered in order to insure complete solder reflow and bonding. Historically, fluxless soldering processes have accomplished the pre-cleaning and post-cleaning steps by employing high temperatures and pressures.
In the alternative, external energy or stimulation may be employed to initiate the deoxidization and cleaning. One method of dry or fluxless soldering, Plasma Assisted Dry Soldering (PAD), has been in existence for several years. While it has been demonstrated that PAD soldering provides an effective method of cleaning, it may not always provide an economical fluxless solution due to equipment costs. In addition, process times and costs may not be compatible with certain applications, for instance, in the high volume assembly manufacturing environment. It is therefore recognized that PAD technology has not adapted to the manufacturing environment well, primarily due the above problems.
Nevertheless, it would be desirable to provide a plasma assisted dry cleaning method which performs cleaning and de-oxidizing steps for conductors. In addition, it would be desirable to provide a plasma assisted dry cleaning method that in addition to cleaning would provide a means for protecting the cleaned and de-oxidized conductor surface for conductors which may be soldered at a later time.
Spiro, U.S. Pat. No. 5,380,557 discloses the formation of Carbon Fluorine compositions as non-wettable coatings for applications where wetting is undesirable. Spiro discloses the formation of Carbon Fluoride compositions in specified conditions to form thermally stable carbon fluoride films, high adhesion coatings, and bulk materials by degradation of vapors comprised of compounds containing fluorine and carbon into fragments, followed by condensation of these fragments onto a substrate, preferably maintained at a temperature lower than the fragments, but generally maintained at a temperature high enough for the fragments to have sufficient mobility and energy to rearrange. Spiro discloses the vaporization of compounds containing fluorine and carbon by direct heating, or entrainment with an inert gas such as argon, nitrogen, helium, and the like. The vapors of compounds containing fluorine and carbon are energized so as to degrade them into fragments. This can be accomplished by (1) passing the vaporized compounds over a hot filament or through a hot furnace of suitable temperature or (2) exposing the compounds to microwave (RF) radiation of a sufficient intensity or (3) exposing the compounds to light of a frequency which initiates photolysis. Following degradation, the fragments are condensed onto a substrate.
Spiro further discloses that the carbon fluoride solid compositions produced employing the invention described provide good coatings with high adhesion to the substrates on which they are formed.
In the context of electronics manufacturing, however, it would be desirable to provide a plasma assisted dry cleaning method that also applies a wettable protective coating to the cleaned and de-oxidized conductor surface for conductors which may be soldered at a later time.
In addition, it would be desirable to provide a plasma assisted dry cleaning method that provides a conductive surface that exhibits acceptable wettability in subsequent solder, reflow and sphere placement processes which may be performed as much as a several hours following treatment. In addition, it would be desirable to provide a plasma assisted dry cleaning method for cleaning reworked or salvaged electronic components, including BGA, .mu.BGA and CSP packages which have re-workable defects such as solder shorts or low volume solder.